function A = prepareA(m, n, h)
%PREPAREA Summary of this function goes here
%   Detailed explanation goes here

A = diag(ones(m*n,1)*-4,0) + ...
    diag(ones(m*n-1,1),-1) + ...
    diag(ones(m*n-1,1),1) + ...
    diag(ones(m*n-m,1),-m) + ...
    diag(ones(m*n-m,1),m);

A = A ./ (h^2);

end

